Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly to an organic light emitting display device capable of achieving an intra-pixel integration design with sufficient storage capcacitance and a method of manufacturing the same.
Discussion of the Related Art
With the recent development of various portable electronic appliances such as mobile communication terminals and notebook computers, demand for flat panel display devices applicable to portable electronic appliances is increasing.
Examples of flat panel display devices being currently researched include a liquid crystal display device, a plasma display device, a field emission display device, and an organic or inorganic light emitting diode display device. In particular, in the case of the organic light emitting display device, the application field thereof is being expanded by virtue of its various advantages such as advanced technologies for mass production, easy implementation of driving means, low power consumption, high picture quality, realization of a large screen, and flexibility.
Such an organic light emitting device includes an organic light emitting diode in each pixel for light emission and a pixel circuit for controlling current flowing through the organic light emitting diode. The pixel circuit typically includes at least two thin film transistors, and a storage capacitor.
Meanwhile, thin film transistors are classified into a thin film transistor having a top gate structure and a thin film transistor having a bottom gate structure.
For formation of thin film transistors (TFTs) having a top gate structure, an amorphous silicon layer is first formed over a substrate. The amorphous silicon layer is then crystallized using an excimer layer and, as such, is formed into a polysilicon layer. A photoresist film (not shown) is subsequently coated over the crystallized polysilicon layer. The photoresist film is subjected to a light exposure and development to form a photoresist film pattern. Using the photoresist film pattern as a mask, the polysilicon layer is then etched, to leave an active layer in regions each corresponding to a desired portion of each pixel. A gate insulating film is then formed to cover the active layer. Gate electrodes are finally formed on the gate insulating film, to correspond to the active layer.
On the other hand, formation of TFTs having a bottom gate structure is carried out in such a manner that formation of an active layer and gate electrodes is reverse to that of the TFTs having a top gate structure. Meanwhile, the crystallization process for crystallizing amorphous silicon into polysilicon is carried out at a temperature of 400° C. or more and, as such, disconnection of the active layer may occur during the crystallization process in the bottom gate structure. For this reason, recently developed organic light emitting display devices prefer TFTs having a top gate structure in which gate electrodes are formed after completion of crystallization, in order to eliminate the problem of active layer disconnection.
Hereinafter, one pixel of a conventional organic light emitting display device including TFTs having a top gate structure will be described with reference to the accompanying drawings.
FIG. 1 is a circuit diagram illustrating one pixel of a conventional organic light emitting display device. FIG. 2 is a cross-sectional view taken along a line passing through a drive transistor and a switching TFT, which are illustrated in FIG. 1.
FIG. 1 illustrates a configuration of a pixel circuit in an organic light emitting display device having a basic structure. The pixel circuit includes a switching TFT ST, a drive TFT DT connected to the switching TFT ST, and an organic light emitting diode OLED connected to the drive TFT DT.
The switching TFT ST is formed at a region where a gate line GL and a data line DL cross each other. The switching TFT ST functions to select a pixel. As illustrated in FIG. 2, the switching TFT ST includes a switching gate electrode SG 10 protruding from the gate line GL, a switching source electrode SS branched from the data line DL, a switching drain electrode SD 45, and a first active layer 60 having a switching channel region.
In this case, the switching channel region, which is designated by reference numeral 60a, is defined by a portion of the first active layer 60 overlapping the switching gate electrode SG. Portions of the first active layer 60 disposed at opposite sides of the switching channel region 60a are doped with impurity ions and, as such, function as a source region 60b and a drain region 60c, respectively. The source region 60b and drain region 60c are connected to the switching source electrode SS and switching drain electrode SD of the switching TFT ST, respectively.
Meanwhile, the drive TFT DT functions to drive the organic light emitting diode OLED of the pixel selected by the switching TFT ST. The drive TFT DT includes a driving gate electrode DG 15 connected to the switching drain electrode SD of the switching TFT ST, a drive source electrode DS included in a reference voltage line RL, a driving electrode pattern DD 55 spaced apart from the drive source electrode DS, and a second active layer 70 having a driving channel region 70a and source and drain regions 70b and 70c respectively connected to the drive source electrode DS and driving electrode pattern DD 55 around the driving channel region 70a. The driving electrode pattern DD of the drive TFT DT is connected to a first electrode of the organic light emitting diode OLED.
The driving gatedriving gate electrode DG 15 is arranged over the switching TFT ST and beneath the drive TFT DT while overlapping the switching drain electrode 45 and driving electrode pattern, respectively. Electrical connection is provided at overlapping portions of the driving gate electrode DG 15 and switching drain electrode 45 and, as such, the drain electrode of the switching TFT ST and the gate electrode of the drive TFT DT are connected.
In addition, a storage capacitor Cst may be defined by the overlapping portions of the driving gate electrode DG 15 and driving electrode pattern 55 of the drive TFT DT.
In the conventional organic light emitting display device, the switching drain electrode SD and driving gate electrode DG, which are formed to have a straight shape on a planar surface, overlap each other on the planar surface for a connection between the drain electrode of the switching TFT and the gate electrode of the drive TFT. The switching drain electrode SD and the driving gate electrode DG have an elongated planar straight electrode shape. In this case, the driving electrode pattern DD 55 used as one electrode of the storage capacitor Cst is beneficially not be connected to the driving gate electrode DG 15 even though the driving electrode pattern DD 55 overlaps the driving gate electrode DG 15 when viewed in plan. Accordingly, at least the driving gate electrode DG 15 and the connection portions of the second active layer 70 and driving electrode pattern 55 are spaced apart from each other when viewed in plane. Since the driving gate electrode DG 15 maintains a planar spacing from the connection portions of the second active layer 70 and driving electrode pattern DD 55, the overlap area between the driving electrode pattern DD 55 and the driving gate electrode DG 15 is small. As a result, the storage capacitance determined by the overlap area may be insufficient.
Meanwhile, development of organic light emitting display devices is being accelerated to satisfy demand for large area and high density displays in accordance with gradual expansion of application fields thereof. In particular, as resolution increases, the size of the unit pixel is decreased. The decrease in unit pixel size means that the space of the unit pixel where TFTs and a storage capacitor are arranged is reduced. In the above-mentioned conventional organic light emitting display device, the pixel size may need to be increased to secure a sufficient storage capacitor area. For this reason, it may be difficult to simultaneously secure both high resolution and sufficient capacity of a storage capacitor.
Meanwhile, the organic light emitting diode OLED is formed through a lamination of a first electrode connected to the drain electrode DD 55 of the drive TFT DT, organic layers including an organic light emitting layer, and a second electrode.
In addition, the inter-pixel position of the organic light emitting diode may vary in accordance with light emission types. For example, in a top emission type, the organic light emitting diode can emit light at the top side thereof, irrespective of overlap thereof with a pixel circuit including light-shielding metal wirings. In a bottom emission type, however, the light-shielding metal wirings may shield emission of light and, as such, the aperture ratio of the organic light emitting diode may be reduced as the area of the pixel circuit increases.
In the above-mentioned conventional organic light emitting display device having a top gate structure, the switching drain electrode and driving gate electrode, which are connected in one direction, are beneficially arranged between the active layers of the switching TFT and drive TFT spaced apart from each other when viewed in plane. For this reason, it may be difficult to achieve a desired intra-pixel circuit integration. Accordingly, the conventional organic light emitting display device having a top gate structure may need to secure a space between intra-pixel circuit regions, which makes it difficult to secure high resolution.
In addition, due to the features of the top gate structure, the gate electrode layer occupying a relatively small area in the pixel area is arranged over the active layer and, as such, it may be difficult to secure an area for a storage capacitor. That is, it may be difficult to secure an area overlapping the gate metal layer having a small area for the storage capacitor to have sufficient capacity.